The invention relates to a capsule for a detector operating in an ultra-high vacuum which is suitable as gamma (.gamma.) spectrometer, particularly for a germanium (Ge) detector.
For the spectroscopy of .gamma.-radiation in many areas of research and in industry, detectors of high purity germanium (HPGe) are utilized. These detectors have very good energy Resolution and a high detection efficiency, properties which facilitate a general application in basic nuclear physics research and in space research but also environmental monitoring, radiation protection, non-destructive material analysis and industrial manufacture monitoring.
In order to become operative, Ge-detectors need to be cooled down to about -190.degree. C. by liquid nitrogen. For this purpose the detectors are installed in a vacuum cryostat which, at the same time, protects the very sensitive surfaces of the detectors. The present cryostat technology makes handling and service of the detectors difficult. It also makes it hard to use the same detector for different measuring tasks.
Detectors which are used as .gamma.-spectrometers and are mounted in capsules are utilized for example for the detection of .gamma.-radiation emitted from fast moving atomic nuclei.
In order to limit the energy broadening of the .gamma.-lines caused by inherent Doppler effects, it is desirable to provide an as small as possible solid angle for the detectors and the detectors should have a high granularity. In the interactions between .gamma.-rays and detector materials, scattering effects occur in addition to the desired photo effect to be detected, particularly the so-called Compton scattering, which provides for a continuous background in the .gamma.-ray spectrum and which deteriorates the detector response function. To minimize this Compton background, the detector is made as large in volume as possible. In addition, an anti-Compton detector is utilized by which the Compton-scattered .gamma.-radiation escaping from the detector can be detected and suppressed.
By arranging the detectors in evacuated capsules, a high reliability of the detectors during experiments can be achieved. The highly sensitive surfaces of the detector crystals are protected by the ultra high vacuum in the chamber. In this way, the handling of the detectors becomes quite simple, particularly during regeneration after radiation damage. Encapsulated detectors can also be advantageously arranged in clusters comprising several detectors so that, by adding the energies deposited in the various detectors in case of Compton scattering, the total energy of a .gamma.-quantum can be determined with high accuracy.
In order to maintain the sensitivity of the detector crystals over long periods of operation, the ultra high vacuum in the detector chamber must be-kept as constant as possible.
It is the object of the present invention to provide a capsule for a detector with a detector chamber in which an ultra high vacuum can be maintained over a long period of operation.